Graphitization of carbon articles



June 30, 1953 v. c. HAMISTER 2,644,020

GRAPHITIZATION OF CARBON ARTICLES Filed March 28, 1950 INVENTOR VICTOR c. HAMISTER ORNEY Patented June 30, 1953 GRAPHITIZATION OF CARBON ARTICLES Victor Carl Hamister, Lakewood, Ohio, assignor to Union Carbide and Carbon Corporation, a

corporation of New York Application March 28, 1950, Serial No. 152,459

8 Claims.

This invention relates to an improved method or baking and graphitizing shaped carbon articles and is particularly useful in the graphitization of furnace electrodes. The invention will be described in detail with respect to the graphitization of furnace electrodes although the invention is not limited to this use.

Conventionally, green electrodes, formed from carbon and a binder, are baked in a gas-fired furnace at a temperature of from 700 C. to 1000 C., and then, after cooling, are transferred to an electric furnace and packed in a granular carbonaceous conductive material which acts as a resistance element. The pre-baked electrodes are heated in the electric furnace to about 2.700 C. and converted to graphite. This is known in the art as the duplex process.

Another process which has been suggested eliminates the preliminary baking in the gas-fired furnace. The green electrodes are heated to graphitization temperatures in one operation in an electric furnace. Again, granular carbonaceous conductive material is used as the resistance element in the electric furnace. The high incidence of broken electrodes occurring in this process detracts from its utility, which is otherwise good because of advantages in savings of fuel, time and equipment.

The granular carbonaceous conductive packing material, such as is used as the resistance element in electric furnaces, has a negative coefficient of resistance, i. e., its electrical resistance decreases as its temperature increases. If for any reason one part of the furnace becomes hotter than other parts, the flow of current through that part increases thereby aggravating the condition. The liberation of heat in the furnace, particularly at temperatures below about 700 C., is not uniform. The large number of broken electrodes obtained in the single heating process is attributable to the fact that green electrodes cannot endure the nonuniform heating of the electric furnace. The pre-baked electrodes which are heated in the electric furnace in the duplex process are stronger and more resistant to the uneven heating.

The primary object of the invention is to provide a method whereby shaped carbon articles, such as green electrodes, can be heated to graphitization temperatures in a single heating operation in an electric furnace without undue breakage.

A further object of the invention is to provide a method of baking carbon articles at temperatures less than that required for their conversion to graphite. More particularly it is an object of the 2 invention to provide a method of baking carbon article at temperatures higher than that which can be maintained satisfactorily in a gas fired furnace but not necessarily high enough to con= vert them to graphite.

The manner in which the objects of the invention are accomplished will be apparent from a description of the graphitization of furnace elec trodes in accordance with the method of the invention.

Green electrodes which are to be graphitized, are baked in an electric furnace which has embedded in a granular carbonaceous conductive packing material an auxiliary resistance element such as a metallic grid. The auxiliary resistance element has a lower resistance than the granular packing at temperatures up to about 600 C. and also has a positive thermal coefficient of resistance. Until the temperature of the furnace reaches about 600 C., the current passes through the auxiliary resistance element. If any part of this element becomes hotter than the rest of the element, its resistance increases and the current is diverted to cooler areas insuring a uniform libcration of heat. After the temperature in the furnace reaches about 600 C., the current is carried by the packing material. Thus, the electrodes are raised evenly to their graphitization temperature without the danger of excessive breakage.

The drawing is illustrative of the manner in which the invention is practiced. In the drawing:

Fig. 1 is a plan view, parts being broken away, of an electric furnace containing electrodes to be heated to graphitization temperatures in accordance with the invention.

Fig. 2 is a view taken along the line 2--2 of Fig. 1.

In preparation for heating, the green electrodes H are placed, preferably upright, in the electric furnace l2. Surrounding the green electrodes H is a granular carbonaceous packing material l3, such as ground coke, in which are embedded metallic grids l4, suitably of chicken wire. The granular material i3 is in close contact with the green electrodes 1 I and the furnace electrodes l5. The metallic grids l4 extend almost the length 01 the furnace l2 terminating near the furnace electrodes it. Best results have been obtained when a rod or bar of graphite i6 is attached to the ends of the grids I4 as, for example, with copper rivets H. The joining of the graphite rods or bars IE to the grid l4 facilitates the flow of current, when it is turned on, from the furnace electrodes [5 to the grids I4, but such an arrangement is optional.

The number and arrangement of the grids I4 will depend in part upon the siZe of the furnace l2 as well as the disposition and size of the green electrodes H being graphitized. In most instances, grids should be above and below as well as on both sides of the green electrodes H, as is illustrated in the drawing.

Preferably, the resistance of the grids l4 should be less than that of the packing material I3 at temperatures up to about 600 C. The resistance of the grids It must be less than that of the packing material H3 at temperatures below 400 (3., otherwise the number of broken electrodes is excessive. In general, a wire netting, commonly called chiclren wire, with up to or openins per foot and a wire diameter of from 10 to mils will serve as a suitable grid.

When the green electrodes H, granular carbonaceous packing material l3, and the metallic grids id have been arranged in the furnace l2, described above, the current is turned on. .Practically all of the current travels through "he grids M which are heated and in turn heat the adjacent packin material ill and the green electrodes i i. i ls the temperatures in the furnace rises, the resistance of the grids it increases and that of the paclcing material i3 decreases. At the temperature at which the resistance of the grids it is of greater than that of the packing material l3, suitably about 660 (3., most of the current through the packing material !3. From this point until the electrodes I I are graphitized the packing material will serve as the primary source of heat in the furnace. Current is sup= plied to the furnace until the graphitization tempearture of about 2,700 C. is reached.

a typical graphitization run in which the initial firing stage was carried up to a temperature of lli00 (3., the time required to bring the furnace up to this temperature was about 250 hours. The high temperature or graphitization in which the temperature was increased to about 270W (3., required about hours.

in addition to furnace electrodes, other shaped articles that can be graphitized according to the invention include carbon plates for motor generator brushes, cylinders and rods for mercury rect .ers, and graphite bars used in electrolytic cells.

It will b understood that the invention is not limited to the graphitization of carbon articles but is also useful when it is desired to bake the carbon article at temperatures less than those at which graphitization occurs. It has been found to particularly advantageous for baking articles at tem ratures above 1500 C. which are cliilicult or impossible to maintain in a gas fired furnace.

I claim:

1. In a process baking shaped carbon articles in electric furnace of the kind having a packing of granular carbon which serves as an electrical heating resistor between the furnace electrodes, which kind of furnace is characterized by slow and non-uniform heating of the packing until the average temperature thereof is about 2 .09 C. and by more rapid and more uniform ll 7 thereabove, the improvement which coinprises embedding an auxiliary resistance element in said granular carbon, said granular carbon having a negative temperature coefficient of resista ce, said auxiliary resistance element having a positive temperature coefficient of resistance and having a resistance lower than that of said packing material at temperatures below 400 C disposing said auxiliary resistance element in said furnace in such a manner that it provides a path for electric current through said furnace at temperatures below 400 C., passing an electric current through said furnace whereby the temperature of said carbon articles is raised substantially uniformly to 400 0., and continuing the flow of current through said furnace to elevate the temperature of said carbon articles above 400 C., the resistance of said packing becoming less than that of said auxiliary resistance during said continued flow of current.

In a process of baking shaped carbon articles in an electric furnace of the kind having a packing of granular carbon which serves as an electrical heating resistor between the furnace electrodes, which kind of furnace is characterized by slow and non-uniform heating of the packing until the average temperature thereof is about 600 C. and by more rapid and more uniform heatin thereabove, the improvement which comprises embedding an auxiliary resistance element in granular carbon, said granular carbon having, nega vs temperature coefficient of resistance, auxiliary resistance element hava positive temperature coeihcient of resistance and having sistance lower than that of said 'ng ,material at temperatures below 6il0 13., disposing auxiliary resistance elet in said furnace in such a manner that it l a path for electric current through said urnace at temperatures below 600 0., passan electric current through said furnace whereby the temperature of said carbon articles raised substantially uniformly to 600 C., and continuing the flow of current through said fu te the temperature of said carbon 6 3* 0., the resistance of said than that of said auxiliary said continued flow of current. a. In a process of baking shaped carbon articles in an electric furnace of the kind having a packing of granular carbon which serves an electrical heating resistor between the furnace electrodes, which lrind of furnace is characterize by slow and non-uniforin heating of the packing until the average temperature thereof is about eiill and by more rapid and more uniform heating thereabove, the improve ment v iich embedding a plurality of metallic in granular carbon, said granular carbon having a negative temperature coefficient resistance, said metallic grids having a pos He temperature coemcient of resistance and 11a a resistance lower than that of said packing material at temperatures below 460 0, disposing metallic grids in said furnace in such a manner that they provide path for electric current through said furnace at temperatures below coo passing an electric current through said furnace whereby the temperature of said carbon articles is raised substantially uniformly to 400 ll, and continuing the flow of current through said furn to elevate the temperature of said carbon articles above 400 6., the resistance of said packing becoming than that of said metallic grids during said continued flow of current.

In a process of bakin shaped carbon articles in an electric furnace of the kind having a packing of granular carbon which serves as an electrical heating resistor between the furnace electrodes, which kind of furnace is characterized by slow and. non-uniform heating of the packing until the average temperature thereof is about 600 C. and by more rapid and more uniform heating thereabove, the improvement which comprises embedding a plurality of metallic grids in said granular carbon, said granular carbon having a negative temperature coefficient of resistance, said metallic grids having a positive temperature coefficient of resistance and having a resistance lower than that of said packing material at temperatures below 600 C., disposing said metallic grids in said furnace in such a manner that they provide a path for electric current through said furnace at temperatures below 600 C., passing an electric current through said furnace whereby the temperature of said carbon articles is raised substantially uniformly to 600 C., and continuing the iiow of current through said furnace to elevate the temperature of said carbon articles above 600 C., the resistance of said packing becoming less than that of said metallic grids during said continued flow of current.

5. In a process of graphitizing carbon elec trodes in an electric furnace of the kind having a packing of granular carbon which serves as an electrical heating resistor between the furnace electrodes, which kind of furnace is characterized by slow and non-uniform heating of the packing until the average temperature thereof is about 400 C. and by more rapid and more uniform heating thereabove, the improvement which com= prises embedding strips of chicken wire in said granular carbon, said granular carbon having a negative temperature coefficient of resistance, said chicken wire having a positive temperature coefiicient of resistance and having a resistance lower than that of said packing material at temperatures below 400 C., disposing said chicken wire in said furnace in such a manner that it provides a path for electric current through said furnace at temperatures below 400 C., passing an electric current through said furnace whereby the temperature of said electrodes is raised substantially uniformly to 400 C., and continuing the flow of current through said furnace to elevate the temperature of said electrodes to their graphitization temperature, the resistance of said packing becoming less than that of said chicken Wire during said continued flow of current.

6. In a process of graphitizing carbon electrodes in an electric furnace of the kind having a packing of granular carbon which serves as an electrical heating resistor between the furnace electrodes, which kind of furnace is characterized by slow and non-uniform heating of the packing until the average temperature thereof is about 600 C. and by more rapid and more uniform heating thereabove, the improvement which comprises embedding strips of chicken wire in said granular carbon, said granular carbon having a negative temperature coefficient of resistance, said chicken wire having a positive temperature coeiiicient of resistance and having a resistance lower than that of said packing material at temperatures below 600 C., disposing said chicken wire in said furnace in such a manner that it provides a path for electric current through said furnace at temperatures below 600 C., passing an electric current through said furnace whereby the temperature of said electrodes is raised substantially uniformly to 600 C., and continuing the flow of current through said furnace to elevate the temperature of said electrodes to their graphitization temperature, the resistance of said packing becoming less than that of said chicken wire during said continued flow of current.

7. In a process of ,graphitizing green carbon electrodes in an electric furnace of the kind having a packing of granular carbon which serves as an electrical heating resistor between the furnace electrodes, which kind of furnace is characterized by slow and non-uniform heating of the packing until the average temperature thereof is about 400 C. and by more rapid and more ui form heating thereabove, the improve ment which comprises embedding strips of chicken wire in said granular carbon, said granular carbon having a negative temperature coefficient f resistance, said chicken wire having a positive .einperature coefficient of resistance and having a resistance lower than that of said packing material at temperatures below 400 C., disposing said chicken wire in said furnace in such a mannor that it provides a path for electric current through said furnace at temperatures below 400 C., passing an electric current through said furnace whereby the temperature of said green electrodes is raised substantially uniformly to 400 C., and continuing the flow of current through said furnace to elevate the temperature of said green electrodes to their graphitization temperature, the resistance of said packing becoming less than that of said chicken wire during continued flow of current.

8. In a process of graphitizing green carbon electrodes in an electric furnace of the kind having a packing of granular carbon which serves as an electrical heating resistor between the furnace electrodes, which kind of furnace is characterized by slow and non-uniform heating of the packing until the average temperature thereof is about 600 C. and by more rapid and more uniform heatin thereabove, the improvement which comprises embedding strips of chicken Wire in said granular carbon, said granular carbon having a negative temperature coefficient of resistance, said chicken wire having a positive temperature coefficient of resistance and having a resistance lower than that of said packing material at temperatures below 600 C., disposing said chicken wire in said furnace in such a manner that it provides a path for electric current through said furnace at temperatures below 600 C., passing an electric current through said furnace whereby the temperature of said green electrodes is raised substantially uniformly to 600 C., and continuing the flow of current through said furnace to elevate the temperature of said green electrodes to their graphitization temperature, the resistance of said packing becoming less than that of said chicken wire during said continued flow of current.

VICTOR CARL I-IAMISTER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 752,357 Price Feb. 16, 1904 840,044 Collins Jan. 1, 1907 865,608 Price Sept. 10, 1907 1,030,620 Sprague et a1 June 25, 1912 1,130,215 Steere Mar. 2, 1915 1,390,823 Sieurin Sept. 13, 1921 1,440,724 Soderberg Jan. 2, 1923 

